1. Field of Invention
The present invention relates to semiconductor device manufacturing and, in particular, to apparatuses and methods for chemical-mechanical polishing of semiconductor wafers.
2. Related Art
FIG. 1 is a side view of a conventional chemical-mechanical polishing (CMP) apparatus used, for example, to planarize the device surface of a processed semiconductor wafer during device fabrication. Circular platen 10 is shown connected to platen drive 12 through shaft 14. Platen drive 12 causes platen 10 to rotate during polishing operations. Polishing pad 16 is shown mounted on platen 10 top surface 18. The top surface of pad 16 is polishing surface 20. Pad 16 is typically felt or urethane with a foam backing, or a material having an abrasive embedded in surface 20.
Also shown is a conventional wafer carrier 22 positioned over platen 10. Carrier 22 is attached to carrier drive 24 through shaft 26. Carrier 22 holds device surface 28 of processed semiconductor wafer 30 against polishing surface 20 during polishing. During some conventional polishing operations, carrier drive 24 causes carrier 22, and hence wafer 30, to rotate against polishing surface 20. In other conventional polishing operations, carrier drive 24 causes carrier 22, and hence wafer 30, to translate laterally across polishing surface 20. In some apparatuses drive 24 may cause simultaneous rotational and translational motion of carrier 22.
Slurry reservoir 32 supplies conventional polishing slurry 34 to polishing surface 20 using conventional slurry delivery system 36. Slurry 34 may be an abrasive carried in a fluid suspension, a compound formulated to have a chemical effect on device surface 28, or a combination of both. Platen 10""s rotation causes slurry 34 to flow radially outward across polishing surface 20, thus creating a thin layer (not shown) of polishing slurry on surface 20. Delivery system 36 typically includes pumps and tubing (not shown).
During conventional chemical-mechanical polishing, slurry 34 is dispensed at a fixed flow rate onto polishing surface 20. Wafer 30 is mounted to carrier 22 which is then positioned to place device surface 28 against polishing surface 20. Platen 10 and carrier 22 are rotated and, as additional slurry 34 is introduced, the desired polishing effect on device surface 28 is obtained. When polishing is completed, carrier 22 is removed from polishing surface 20 and wafer 30 is removed from carrier 22. A new wafer may then be mounted on carrier 22, and the polishing process is repeated.
The polishing operation adversely affects polishing surface 20. The pressure of device surface 28 against polishing surface 20 typically deforms surface 20""s fine surface structure. Therefore, a conditioner is typically used either during or after polishing so as to keep surface 20 in a near pristine state. The conditioner is typically a compound of diamonds that are bonded to a substrate. The bonded diamonds and substrate are then nickel plated, and the combination is used to clean and roughen the polishing pad surface. The conditioning process is difficult to optimize, however, and consequently it is difficult to keep polishing surface 20 in the required state for proper polishing. What is required is a chemical-mechanical polishing apparatus and process in which the polishing surface is easily kept in a near optimum state for polishing.
In accordance with the invention, a chemical-mechanical polishing (CMP) apparatus is configured with an aggregate polishing disk mounted on a rotating platen. The aggregate polishing disk includes abrasive particles held in solid suspension by a radiant energy sensitive bonding agent. In some embodiments the abrasive particles are of silicon oxide (silica), aluminum oxide, or cerium oxide, and the bonding agent is a conventional positive photoresist that is sensitive to ultraviolet (UV) light. The aggregate polishing disk has a flat top surface.
The CMP apparatus also includes a fixture holding a radiant energy source for emitting energy to which the bonding agent is sensitive, such as a UV light source. In some embodiments the UV light source is a plurality of conventional UV producing light bulbs, and in other embodiments a single conventional long UV producing bulb is used. The UV light source fixture is configured with a slot so that UV light from the UV light source shines through the slot. The fixture is positioned over the aggregate polishing disk""s top surface so that the slot is approximately aligned with a radius of the disk. Thus, UV light from the UV light source is incident on the aggregate polishing disk top surface through the slot. In some embodiments the slot is a tapered shape so that an approximately equal amount of UV light is incident per unit area of the top surface as the rotating top surface passes beneath the slot. A thin layer of the positive photoresist underneath the top surface is exposed by the incident UV light.
In addition, the CMP apparatus comprises a reservoir containing a developing fluid and a conventional dispensing mechanism to dispense the developing fluid to the aggregate disk""s top surface. Some embodiments of the invention use potassium hydroxide (KOH) or ammonium hydroxide (NH4OH) as the developing fluid, although other compounds may be used. In some embodiments of the invention a second reservoir containing a rinsing fluid and a second conventional dispensing mechanism is added. Some embodiments of the invention use deionized water as a rinsing fluid.
In accordance with the invention, a semiconductor wafer may be processed as follows. The aggregate polishing disk is rotated and the top surface receives UV light through the slot in the UV fixture. The UV light exposes a thin top portion of the aggregate disk as the disk rotates. After exposure, developing solution is dispensed onto the aggregate disk""s top surface and is dispersed into a thin film by the disk""s rotation. The developing fluid develops and dissolves the thin layer of UV-exposed positive photoresist, thus releasing the abrasive particles within the UV-exposed layer. Thus a polishing slurry of developing fluid and abrasive particles is created. The surface underlying the developed thin layer acts as a polishing surface.
A processed semiconductor wafer is mounted to a conventional wafer carrier and is held against the polishing surface of the rotating aggregate disk such that a thin film of polishing slurry exists between the polishing surface and the wafer""s device surface. In some embodiments the polishing surface continually receives UV light and developing fluid during polishing. The constant UV exposure and developing of a thin top portion of the polishing disk ensures adequate slurry is available for polishing. In some embodiments the amount of UV exposure and dispensed developing fluid is controlled so as to control the resulting amount of polishing slurry. When polishing is complete the wafer is removed, rinsing fluid is dispensed onto the polishing surface to remove the used polishing slurry, and the polishing process may be repeated for another processed wafer. No polishing surface conditioning is required.